The present invention relates to a testing or measuring instrument.
More particularly, it relates to a test or measuring probe for monitoring the level of liquid in storage containers.
Still more specifically, the invention herein relates to a test probe for storage containers of the type adapted to hold flammable liquids.
Test probes of this general type are known from German published Application DE-OS No. 2,944,076. The probe disclosed in that application is intended to continuously monitor the level of liquid in a container, for which purpose it utilizes a resistance wire that is stretched in U-shaped configuration over the test path and produces a resistance value proportional to the liquid level in the container, via a short-circuiting element, such as a slider.
The known probe is generally capable of fulfilling its basic intended purpose. However, it is possessed of certain drawbacks. For example, over time the accuracy and stability of the measured values are not adequate to meet the requirements made in modern applications. One important reason for this resides in the use of the resistance wire itself, since the wire may tear or elongate and, in either event, will then make electrical contact with the guide tube which will result in predictable difficulties. Furthermore, a resistance wire as used in the prior art has no means of temperature compensation nor any facility for monitoring the wire and/or its performance. The small contact forces exerted by the tactile elements of the device may result in time-dependent contact and transition resistances whose development then leads to erroneous measuring results. Given these factors it has been found that the prior art system does not meet certain governmental specifications, such as for example that promulgated in the Federal Republic of Germany in the Calibration Standard for "Measuring Devices for the Volume Measurement of Liquids".
Another prior art drawback is that the resistance wire is stretched, i.e. mechanically stressed. Such a wire can be used only with guide tubes having a maximum length of 3 meters; beyond this, safety considerations preclude the use of these wires. Also, it is not possible in the prior art to employ electrical balancing in order to match the resistance wire to the nominal container filling line, which is another self-evident drawback.
The prior art probe is kept afloat in the container by foldable floats which, to reduce their weight, are hollow and thin-walled. Over time this construction results, however, in a weight-increase of the probe, due to diffusion of the liquid container contents into the floats. Added to this is the likelihood--usually unavoidable--that the device will encounter significant buoyancy resistance (e.g. due to the presence of particulate contaminants in the liquid, or the presence of solder or weld seams or spots in the longitudinal direction of the guide tube) which may cause the floats to be snagged or otherwise to be kept from rising and descending with the liquid level; it goes without saying that this will result in erroneous measurements. There being insufficient buoyancy, it is clear that the prior art is not suitable for use in connection with liquid media having a low specific gravity, such as for example supertype gasoline. It is, of course, also very definitely not usable in pressurized containers since the floats are too fragile to withstand container pressurization.
And finally, the prior art is suitable only for applications in which the cover of the container has--and for structural and other reasons can have--an opening with an inner diameter of two inches or more. The device must not be allowed to touch the surrounding elements, so that the opening cannot be made any smaller for use with the existing device. On the other hand, however, it is not feasible to make both the opening and the device itself any smaller, since this would require a reduction in the size of the floats and would further impair their already inadequate buoyancy.